This invention relates to a method for nondestructive material testing with ultrasound pulses which are introduced into the work piece under test and are reflected by a fault, the position of which is derived from the propagation time of the ultrasound pulses and the echo pulses.
In nondestructive material testing, methods which are based on the scattering or reflection of ultrasound waves in materials and which allow the detection and localization of changes in the structure, especially of faults are already known. Ultrasound is directionally coupled into the surface of the workpiece under test and is displaced on the surface along the workpiece under insonification conditions which remain constant. The echo scatter radiation from the workpiece is picked up and the amplitude of the successive echo pulses is made visible on a picture screen. During the spatial displacement, the amplitude changes of the echo pulses resulting from the different successive ultrasound pulses are recorded or stored. On the picture screen, the amplitude of the echo pulses can be made visible as a function of the lengthwise displacement of the ray bundle. In the arrangement for implementing the method, the transmitters for the ultrasound pulses can, at the same time, be used as receivers for the echo pulses. (German Offenlegungsschrift No. 2 600 720).
In another method for the continuous testing of solid bodies by means of ultrasound, the workpiece is immersed in a liquid coupling medium and the result of the test is likewise displayed by means of a cathode ray tube having brightness modulation. To take into consideration the influence of propagation paths of different lengths for the ultrasound pulses in a coupling medium on the one hand and in the workpiece on the other hand, the time interval between the emission of an ultrasound pulse and its arrival at a point of incidence of the body under test is determined by means of an echo signal assigned to this point of incidence (German Offenlegungsschrift No. 16 98 518). However, testing of relatively large work pieces requires a correspondingly large expenditure. To this disadvantage is added the fact that all liquid coupling media generally have a substantially lower acoustic impedance than the material under test.
The coupling becomes particularly difficult if numerous transducer elements are combined in a so-called array. Therefore, materials testing is limited to small arrays with, say, 20 oscillators. In a sector manner, focusing and sluing can be obtained by phase-shifted steering, so that a certain angular range can be scanned. However, even with the sector scanner it is only possible to generate a cross-sectional picture, i.e., a two-dimensional picture. Large arrays are impossible because of the coupling problems to predetermined curved surfaces.
It is now an object of the present invention to describe a method for nondestructive material testing with ultrasound which allows finding a fault and imaging it, in addition, three-dimensionally in a relatively large volume, approximately 0.5.times.0.5.times.0.5 m.sup.3, nearly independently of the surface structure and form. Finding and imaging the fault should be possible in a short time, preferably substantially less than one minute.